Synergistic Effect of MoS2 Nanosheets and VS2 for ... - ACS Publications

Nov 9, 2017 - efficiency and nonpollution to solve the energy crisis.1 The catalyst is the key ... energy of hydrogen atoms on the edge of layered MoS...
0 downloads 0 Views 7MB Size
Download PDF
Research Article www.acsami.org

Synergistic Effect of MoS2 Nanosheets and VS2 for the Hydrogen Evolution Reaction with Enhanced Humidity-Sensing Performance Xiaofan Chen,† Ke Yu,*,†,‡ Yuhao Shen,† Yu Feng,† and Ziqiang Zhu† †

Key Laboratory of Polar Materials and Devices (Ministry of Education of China), Department of Electronic Engineering, East China Normal University, Shanghai 200241, China ‡ Collaborative Innovation Center of Extreme Optics, Shanxi University, Taiyuan, Shanxi 030006, China

Downloaded via KAOHSIUNG MEDICAL UNIV on October 11, 2018 at 13:07:08 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

S Supporting Information *

ABSTRACT: As a typical transition-metal dichalcogenides, MoS2 has been a hotspot of research in many fields. In this work, the MoS2 nanosheets were compounded on 1T-VS2 nanoflowers (VS2@MoS2) successfully by a two-step hydrothermal method for the first time, and their hydrogen evolution properties were studied mainly. The higher charge-transfer efficiency benefiting from the metallicity of VS2 and the greater activity due to more exposed active edge sites of MoS2 improve the hydrogen evolution reaction performance of the nanocomposite electrocatalyst. Adsorption and transport of an intermediate hydrogen atom by VS2 also enhances the hydrogen evolution efficiency. The catalyst shows a low onset potential of 97 mV, a Tafel slope as low as 54.9 mV dec−1, and good stability. Combining the electric conductivity of VS2 with the physicochemical stability of MoS2, VS2@MoS2 also exhibits excellent humidity properties. KEYWORDS: MoS2, VS2, VS2@MoS2, nanomaterials, hydrogen evolution reaction, humidity sensor

■

INTRODUCTION Electrocatalytic splitting of water through the hydrogen evolution reaction (HER) is a promising method for its highefficiency and nonpollution to solve the energy crisis.1 The catalyst is the key factor influencing the speed or efficiency for the HER, which requires low overpotential and high stability. Because of the high cost of precious metals with high catalytic activity, the replacement of them has been investigated all over the world. Two-dimensional transition-metal dichalcogenides (TMDs) are inexpensive and abundant candidates for the HER. Owing to the alternating sheet structure with a transition-metal atom layer sandwiched between two chalcogen atom layers, all catalytically active edge sites are exposed, which is the critical advantage to improve HER properties.2 As a typical transition-metal sulfide, MoS2 with a bandgap range from 1.2 to 1.8 eV as the thickness decreased to the monolayer has been developed into an ultrasensitive and broadband photodetector3 and a floating gate memory-based monolayer MoS2 transistor.4 More than that, MoS2 also has excellent chemical stability and potential hydrogen evolution activity, and the theoretical calculations show that the binding energy of hydrogen atoms on the edge of layered MoS2 is close to that of platinum.5 MoS2 can be divided into three types of atom arrangements (1T-MoS2, 2H-MoS2, and 3R-MoS2) based on different crystal structures.6 2H and 3R phases have trigonal prismatic coordination and perform as a semiconductor. Although 1T phase with trigonal antiprismatic (or octahedral) coordination is metallic,7 1T-MoS2 is demonstrated to exhibit excellent HER © 2017 American Chemical Society

catalytic performances because of active sites both on the edges and on the basal plane.8 However, 1T-MoS2 and 3R-MoS2 are both metastable.6 Therefore, most attention has been concentrated on 2H-MoS2 because it is stable and easy to obtain. Because of the van der Waals force between the two layers, MoS2 nanosheets are easy to reunite in the process of synthesis, and their conductivity is poor, which hinders the further improvement of hydrogen evolution performance. In the past decade, the main research directions of the catalysts for the HER of MoS2 are to improve catalytic activity by thinning layers9,10 and defecting11−13 or to enhance electron transport by doping14,15 and designing synergetic composites.16−18 The last method is more common and easy to achieve. Chen et al. grew MoS2 on porous MoO2 via the hydrothermal method to enhanced conductivity, which achieved a large current density (10 mA cm−2 at −0.24 V) and a small Tafel slope of 76.1 mV dec−1.19 Liu et al. combined MoS2 nanoparticles with mesoporous graphene, exhibiting excellent electrocatalytic activity and fast charge-transfer kinetics.20 Both theoretical and experimental studies proved that the choice of substrates could influence the energy of hydrogen adsorption.21,22 Different from semiconducting MoS2, VS2, as another typical TMD without bandgap, has shown excellent metallicity in the application of supercapacitors23 and moisture Received: October 1, 2017 Accepted: November 9, 2017 Published: November 9, 2017 42139

DOI: 10.1021/acsami.7b14957 ACS Appl. Mater. Interfaces 2017, 9, 42139−42148

Research Article

ACS Applied Materials & Interfaces Scheme 1. Scheme for the Fabrication of VS2@MoS2

sensors.24 Hydrogen evolution properties of VS2 have already been studied as well.25 Because of good conductivity and active sites, layered VS2 has great potential in the HER. In this work, to compensate for the poor conductivity and expose more active sites on the edges of 2H-MoS2, we developed an active catalyst for the HER of MoS2 nanosheets via a modified two-step hydrothermal method on 1T-VS2 nanoflowers for the first time (Scheme 1). On the one hand, with MoS2 nanosheets growing vertically on the surface, VS2 can prevent the cluster of MoS2, effectively resulting in the formation of thinner nanosheets, so that more active sites can be exposed. The S of VS2 can also be as active sites to absorb and transport the intermediate hydrogen atom, and its basal plane exhibits unusual self-optimizing performance as they catalyze hydrogen evolution.26 On the other hand, the highly conductive VS2 nanoplates contribute to the high electrical conductivity of VS2@MoS2 and facilitate a fast electron-transfer process by enhancing the electrical contact between the active sites and the electrodes.27 Besides, VS2 nanoflowers with the stable structure can provide stable and shape-controlled skeleton, inhibiting the decomposition of MoS2 on the surface which will contribute to the structure stability of active electrocatalysts.28,29 On the basis of the designed experiment mentioned above, improved HER properties of the VS2@MoS2 catalyst have been obtained with a lower onset potential and Tafel slope, and the stability is optimized as well which is much better than that of pure VS2 and MoS2. First-principle calculations were also implemented to analyze the HER activity of VS2@MoS2. The results of the calculated electronic structure prove that near the Fermi level, electrons can be transferred from VS2 substrates to MoS2-edge because of the interband charge transport which is related to the quantum tunneling toward the interlayer barriers. In addition, we also reported the humidity-sensing properties of VS2@MoS2. The humidity sensor conducts the current in the form of a hydronium ion and, consequently, moisture stimuli are converted into electric signals. As mentioned above, the conductivity of the composite and the ability to absorb hydrogen ions have been improved. Because of the high electropositivity of V4+, VS2 ultrathin nanosheets were verified to be a good humidity sensitive material.24 VS2 can be oxidized easily with poor physicochemical stability, especially in a humid environment, whereas MoS2 with good stability could isolate VS2 from oxygen to keep the metallicity of VS2. Moreover, MoS2 owns outstanding virtues, such as a great surface-area-tovolume ratio, high carrier mobility, and low noise level. MoS2 nanosheets act as an anchor in the VS2@MoS2 hybrid and play a dominant role in eliciting the sensor response.30 Experiments have shown that the humidity-sensing properties and stability of the VS2@MoS2 composite are much better than pure MoS2 and VS2. With the application of 2D materials in flexible electronics, we use a polyethylene terephthalate (PET) film as

the substrate instead of the traditional ceramic substrate, which will have a good prospect in the wearable domain.

■

EXPERIMENTAL SECTION

Synthesis of VS2 Nanoflowers. Typically, 0.7 g of ammonium vanadate (NH4VO3) was added into 54 mL of solution containing 45 mL of deionized water and 9 mL of ammonia (NH3·H2O). After magnetic stirring for 15 min, 2.4 g of thioacetamide was added to the above-mentioned solution as the S source, followed by another 15 min stirring until all the reactants dissolved. The homogeneous solution was transferred into a 100 mL Teflon-lined stainless-steel autoclave and heated at 160 °C for 20 h. The pure black VS2 precipitates were gathered by ultrasonication (in ice water) and centrifugation, washed with distilled water or ethanol, and dried at 60 °C at 6 h in a vacuum drying oven. Preparation of VS2@MoS2 Nanocomposites. Take 0.2 g of the as-prepared VS2 black powder into beaker A with 40 mL of distilled water for 30 min ultrasonication in ice water to prevent oxidation. At the same time, 0.5 g of sodium molybdate dihydrate (Na2MoO4· 2H2O) and 0.8 g of thiourea (CH4N2S) were solved into beaker B with 30 mL of distilled water using 0.4 g of oxalic acid to regulate the PH value followed by magnetic stirring for 20 min. Then, the suspension in beaker A was poured into beaker B to be stirred for intensive mixing. The mixed liquid was transferred into the 100 mL autoclave heated at 200 °C for 24 h. By the same cleaning step, the VS2@MoS2 nanocomposite was eventually gathered after being dried at 60 °C at 6 h in a vacuum drying oven and annealed at 550 °C for 1 h with protecting gas to increase the crystallinity before further characterization. Characterization. X-ray diffraction (XRD) was measured by Bruker D8 ADVANCE diffractometer using monochromatized Cu Kα radiation with a λ of 1.5418 Å. Raman spectra were acquired on a Jobin-Yvon LabRAM HR 800 micro-Raman spectrometer. X-ray photoelectron spectra (XPS) were performed on a Kratos Axis ULTRA X-ray photoelectron spectrometer with monochromatic Al Kα radiation. Field-emission scanning electron microscopy (FE-SEM) measurements were carried out on a JEOL JSM-6700F SEM, and a JEOL 2010 field-emission electron microscope at an acceleration voltage of 200 kV was employed to get the transmission electron microscopy (TEM) images. Ultraviolet photoelectron spectroscopy (UPS) and near-edge X-ray absorption fine structure (NEXAFS) experiments were performed at the Catalysis and Surface Science Endstation at the BL11U beamline in the National Synchrotron Radiation Laboratory (NSRL) in Hefei, China. A sample bias of −5 V was applied to observe the secondary electron cutoff with a photon energy of 40 eV. The work function (ϕ) can be determined by the difference between the photon energy and the binding energy of the secondary cutoff edge. The S L-edge NEXAFS was measured in a total electron yield mode. Calculations. The first-principles calculations were performed by Vienna Ab initio Simulation Package with the projector-augmented wave. The generalized gradient approximation in the scheme of the Perdew−Burke−Ernzerhof was used for the exchange−correlation functional. The energy cutoff was set to 500 eV, and a Monkhorst− Pack k-point mesh of 5 × 5 × 1 was used during all supercell calculations. The residual forces for each ion converged less than 0.02 eV/A after structure optimization. Band structure calculations were 42140

DOI: 10.1021/acsami.7b14957 ACS Appl. Mater. Interfaces 2017, 9, 42139−42148

Research Article

ACS Applied Materials & Interfaces

Figure 1. (a) XRD patterns of VS2 and VS2@MoS2. (b) Raman spectra of MoS2, VS2, and VS2@MoS2. High-resolution XPS spectra: (c) V 2p, (d) Mo 3d, and (e) S 2p. performed along the paths, connecting the high-symmetry points: G (0, 0, 0), K (−1/3, 2/3, 0), and M (0, 0.5, 0) in the k-space. Electrochemical Measurements. VS2@MoS2 black powder (2 mg) and 20 μL of Nafion solution (5 wt %, DuPont) were dispersed in 400 μL of an isopropanol/water mixed solvent with the volume ratio of 1:3 by sonication for 30 min to form a homogeneous ink. Then, 10 μL of the dispersion was loaded onto a glassy carbon electrode (3 mm diameter) and dried at 40 °C in vacuum. The electrochemical measurements were performed in a three-electrode system at room temperature by a CHI660D electrochemical workstation with a Pt wire as the counter electrode, Ag/AgCl as the reference electrode, and the as-prepared glassy carbon electrode as the working electrode. All three electrodes worked in an N2-saturated 0.5 M H2SO4 electrolyte. Fabrication and Measurement of the Humidity Sensor. A small amount of the sample as a moisture-sensitive active material was added into the centrifuge tube (1.5 mL) and mixed with a few drops of ethanol. After ultrasonic treatment, until the sample evenly dispersed, the obtained slurry was spin-coated onto a PET flexible substrate with Ag interdigitated electrodes (inset of Figure 7a) and letting it air-dry. The humidity sensor worked in a closed glass vessel where humidity environments were controlled by saturated salt aqueous solutions [LiCl, MgCl2, Mg(NO3)2, NaCl, KCl, and KNO3, which yielded 11, 33, 54, 75, 85, and 95% relative humidity (RH), respectively]. The measurements were carried out on a CHS-1 intelligent humiditysensitive system.

good stacked structures; however, compared with the pure MoS2, the degradation of (002) peaks is likely due to the decrease of MoS2 layers. The Raman spectra (Figure 1b) reveals that the nanocomposites display four main peaks containing two peaks of VS 2 at 280 and 404 cm −1 corresponding to the vibration Eg mode and Ag mode (out of plane) and two peaks of MoS2 at 381 and 407 cm −1 corresponding to the in-layer E12g vibration mode and out-ofplane A1g vibration mode. XPS was performed to explore the elemental composition and the chemical state of VS2@MoS2. Figure 1c shows that two characteristic peaks located at 523.7 and 516.3 eV arose from V4+ 2p1/2 and V4+ 2p3/2 orbitals, indicating the oxidation state of V. As shown in Figure 1d,e, the peak-fitting analysis of Mo4+ 3d3/2 (232.5 eV) and Mo4+ 3d5/2 (229.3 eV) orbitals confirms the presence of Mo4+, and the signal peaks at 163.9 and 162.7 eV are assigned to S2+ 2p1/2 and S2+ 2p3/2 levels. In addition, S L-edge NAXAFS spectra of the VS2@MoS2 hybrid in Figure S1 indicates that MoS2 and VS2 are not stacked on the macroscopic scale but form a heterojunction with high lattice match, resulting in a bonding effect between two atomic layers of two materials. It can be interpreted that the 4d orbit of an Mo atom will be out of plane interacting with the S 2p orbit of VS2. At the same time, the 3d orbit of a V atom will be out of plane interacting with the S 2p orbit of MoS2. Figure S1 also shows that some unsaturated S2− exist, which is beneficial for the HER. In addition, we also calculate the work function of MoS2 and VS2 through the UPS test to support the electron transfer. The results are shown in Figure S2. To confirm the direction of electron transfer, XPS spectra of Mo 3d in pure MoS2 are compared with those of Mo 3d in VS2@MoS2, which is added to Figure S3 in the Supporting Information. It is known that electrons transferred to the d orbit of the Mo atom will result in the shift of Mo 3d3/2 and Mo 3d5/2 peaks to lower binding energy. Therefore, the observed peak shift of VS2@MoS2 in Figure S3 indicates that the electrons are transferred from VS2 to MoS2.

■

RESULTS AND DISCUSSION As the XRD of as-prepared VS2, MoS2, and VS2@MoS2 shown in Figure 1a, pure VS2 and MoS2 are assigned to hexagonal VS2 (JCPDS-36-1139) and the hexagonal phase of MoS2 (JCPDS 37-1492), respectively. The feature peaks of VS2 locate at 15.45°, 35.77°, and 45.26°, respectively, corresponding to the crystal faces (001), (101), and (102). The main peaks of MoS2 are observed at 14.38°, 33.5°, and 59.06°, matching the crystal faces (002), (101), and (110). The (002) diffraction peak is high and sharp, indicating that MoS2 has a good crystallization and a good layered structure. For the composites, there is no high-indexed diffraction peaks, indicating that the short-range structure distribution of MoS2 nanosheets contributes to more active sites. The strong (002) peak of VS2@MoS2 represents 42141

DOI: 10.1021/acsami.7b14957 ACS Appl. Mater. Interfaces 2017, 9, 42139−42148

Research Article

ACS Applied Materials & Interfaces

Figure 2. (a) Medium-magnification and high-magnification. (Inset) SEM image of VS2 nanoflowers. (b,c) HRTEM image of the VS2. (d) HRTEM image of VS2. (Inset) Intensity signals along the red-dotted lines.

Figure 3. (a−c) SEM image of VS2, MoS2, and VS2@MoS2. (d) TEM image of VS2@MoS2. (Inset) SAED pattern of VS2. (e) HRTEM image of VS2@MoS2. (Inset) HRTEM image of VS2. (f) TEM image of VS2@MoS2. (g−i) Elemental mapping of V, S, and Mo.

inset of Figure 2d). According to the Ostwald ripening process known for the growth of the flowerlike metal sulfide structure, the formation of flowerlike VS2 probably involves two steps: initial nucleating and crystal growth.31 First, the functional groups −NH2 and −SH react with V ions dissociating from NH4VO3 in the reaction vessel to form V−S complexes followed by decomposing to shape VS2 nuclei for further growth. Then, the flowerlike structure formed by VS 2 nanoplates weakly stacks together and self-assembles. To unstack the layers, ammonia was added into the reactants forming NH3-intercalated VS2, which could thin the VS2 nanoplates, and no other ions are residual in the assembled structures after the evaporation of NH3.

FE-SEM and TEM images of VS2 nanoflowers are shown in Figure 2. Figure 2a displays that flower-like VS2 is stacked by a large number of VS2 nanoplates in different directions. The radius of a single VS2 nanoflower and the average thickness of the nanoplate are approximately 8 μm and 100 nm (seen from the inset of Figure 2a), respectively. HRTEM was further performed to confirm the crystallinity of VS2. Figure 2b shows VS2 nanoplates with a d-spacing of ∼5.76 Å corresponding to the (001) plane. In Figure 2c,d, each facets of (102) and (101) can be well-indexed, and the crystallinity of the (101) facet is better than the (102) facet, which is consistent with the result of XRD. To reduce errors, the atomic spacing of VS2 was obtained from the average values of several intensity peaks (the 42142

DOI: 10.1021/acsami.7b14957 ACS Appl. Mater. Interfaces 2017, 9, 42139−42148

Research Article

ACS Applied Materials & Interfaces

Figure 4. (a) HER polarization curves of various samples as indicated. (b) Corresponding Tafel plots. (c) Nyquist plots of MoS2 and VS2@MoS2. (Inset) A common R−C equivalent circuit. (d,e) Cyclic voltammetry curves of MoS2 and VS2@MoS2 at various scan rates in the region of 0.15−0.35 V vs RHE. (f) Linear fitting for the capacitive currents of MoS2 and VS2@MoS2 vs scan rates.

elements of V, S, and Mo are distributed uniformly on the nanocomposites. Electrochemical measurements were carried out on a typical three-electrode system (Figure S5). H2 bubbles are easy to produce on the surface of the glassy carbon work electrode with active materials, and the speed of hydrogen production is also considerable here. Comparative studies were performed on VS2, MoS2, and VS2@MoS2 nanocomposites. As shown in Figure 4a, both the pure VS2 and MoS2 show a poor HER activity with a high onset potential of 602 and 327 mV, respectively. In contrast, VS2@MoS2 exhibits a better HER property. More HER active sites are exposed because of the unique growth pattern, and high temperature annealing results in less impurities and higher crystallinity so that the onset potential of VS2@MoS2 decreases down to 97 mV. Moreover, as the conductive substrate, VS2 provides abundant electrons for the H+ combined with −S in MoS2 via interlayer electron transfer, and with the increase of the overpotential, VS2 gradually has a contribution to the HER itself for which the nanocomposites still keep a low overpotential of 177 mV when the current density reaches to 10 mA cm−2, whereas for the two pure samples, a high overpotential need to be attained at 979 and 611 mV. To set a control experiment, we also prepared the 40% commercial Pt/C electrocatalysts for the referential measurements with almost no onset potential. For the further analysis, Tafel slopes of all the samples were derived from linear sweep voltammetry (LSV) on the Tafel equation η = b log(j/j0) (η is

After hybrid, the morphology structure and composition distribution of the nanocomposite was observed in Figure 3. As Figure 3a−c shows, the structure of VS2 nanoflowers remains the same, which provides a stable skeleton for MoS2. Aggregation occurs on the pure MoS2 nanosheets, leading to countless nanospheres because the pure MoS2 2D nanopetals will curl freely to a closed structure and eventually form surface petals to reduce dangling bonds and reduce surface energy. As for the composite, in the process of sodium molybdate being reduced to MoS2, VS2 acts as a template so that MoS2 nanosheets grew on the surface of VS2 uniformly and vertically. The morphology of VS2@MoS2 is further displayed via TEM detection. A low-magnification TEM image (Figure 3d) shows the full view of a composite structure. The selected area electron diffraction (SAED) (inset of Figure 3d) shows a reciprocal lattice of a hexagonal crystal projected along the (001) direction, which demonstrates that the VS2 thin films stack along the c axis. Moreover, a large number of MoS2-edge sites can be seen, which are beneficial for the HER electrochemical catalysis. The high-magnification TEM image (Figure 3e) shows that the lattice spacing d = 0.62 nm, which means that it coincides well with the crystal face in the MoS2 (002) plane. To further demonstrate the interfacial microstructure features of composites, we provide HRTEM for composites in Figure S4. Energy-dispersive X-ray elemental mapping images (Figure 3f−i) demonstrate that all the 42143

DOI: 10.1021/acsami.7b14957 ACS Appl. Mater. Interfaces 2017, 9, 42139−42148

Research Article

ACS Applied Materials & Interfaces Table 1. Comparison of HER Activity Data among Various Catalysts catalyst

loading amount (mg cm−2)

onset potential (mV)

Tafel slope (mV dec−1)

overpotential/current density (mV)/(mA cm−2)

references

0.285 0.285 0.285 0.22 0.285 0.285

119 160 120 130 110 104 126 97

140 94 50 69 52 76.1 90 54.9

∼400/10 200/13 ∼240/10 ∼235/10 240/10 244/10 177/10

9 10 11 14 32 19 33 this work

one layer MoS2 t-Bu−Li exfoliated MoS2 defect-rich MoS2 V0.09Mo0.91S2 MoS2@Fe3O4 MoS2/MoO2 Cu−MoS2/RGO VS2@MoS2

Figure 5. (a,b) Stability test of VS2@MoS2 and MoS2 through potential cycling, polarization curves before and after 1000 potential cycles, and (c) potential vs time at a constant current density of 20 mA cm−2 for 20 h.

the overpotential, b is the Tafel slope, j is the current density, and j0 is the exchange current density). As can be seen from Figure 4b, the Tafel slope of VS2@MoS2 (54.9 mV dec−1) is nearest to that of 40% commercial Pt/C electrocatalysts (35.4 mV dec−1), indicating the highest efficiency of the catalytic reaction. By contrast, high values of 133.7 and 105.6 mV dec−1 are obtained for the pure VS2 and MoS2 catalysts, respectively. The HER properties of the MoS2-based hydrogen evolution cathode materials in previous report are listed for comparison (see Table 1). In acidic electrolytes, the HER is mainly influenced by two processes.18 First is a discharge process that the hydrogen proton integrates with an electron to form an intermediate-state adsorbed hydrogen atom on the surface of the catalyst called the Volmer reaction H3O+ + e− → Hads + H 2O

(1)

2.3RT ≈ 120 mV αF

(2)

b=

The obtained Tafel slope of the composite catalyst in our work is 54.9 mV dec−1, suggesting Volmer−Heyrovsky HER processes. There may be two reasons for the experimental value higher than the theoretical value; one is that when the surfaceadsorbed hydrogen atom reaches a certain coverage rate, the discharge process will be limited, and the other is the hydrogen produced during the reaction hindered the contact between the catalyst and the electrolyte, which could be proved by the inset of Figure S5. In addition, some external experimental errors such as ambient temperature and electrolyte concentration also influence the results. Electrical impedance spectroscopy was employed to learn the electrode kinetics of MoS2 and VS2@MoS2 catalysts for the HER. As shown in Figure 4c, the semicircles reflect the chargetransfer resistance (Rct) between the electrode and the electrolyte. The R−C equivalent circuit (inset of Figure 4c) was used to research the kinetic differences of different catalysts. The electrical resistance of VS2@MoS2 decreases a lot compared with MoS2 catalysts, suggesting higher conductivity and more active edge sites. In addition, cyclic voltammetry measurements with different scanning rates were introduced to characterize the charge storage capacity and the effective reaction area of the catalysts, as shown in Figure 4d,e. Through the calibration of differential output currents at 0.25 V, Figure 4f was gained according to the current−voltage relationship. Calculation shows that Cdl of VS2@MoS2 and pure MoS2 are 15.02 and 7.13 mF cm−2, respectively, which demonstrates that the double-layer capacitance of the composite materials increases obviously (more than one time); by contrast, pure MoS2 shows a low double-layer capacitance. By compounding, MoS2 can achieve greater surface activity, and as a result, electrons can be stored more and transferred faster, conducing to the improvement of HER performance. Besides the HER activity, stability is also a significant evaluation for the catalysts. The long-time cycling was adopted to test the stability of the catalysts presented in Figure 5. As can

This process occurs on VS2 and MoS2 at the same time, while the adsorbed hydrogen atoms are instable and most of them are transferred onto MoS2. The results of LSV and the Tafel slope of VS2 were conformed. Next is the desorption process of hydrogen including either an electrochemical desorption step called the Heyrovsky reaction H3O+ + Hads + e− → H 2 + H 2O

b=

2.3RT ≈ 40 mV (1 + α)F

(3)

(4)

or a recombination step called the Tafel reaction Hads + Hads → H 2

b=

2.3RT ≈ 30 mV 2F

(5)

(6) 42144

DOI: 10.1021/acsami.7b14957 ACS Appl. Mater. Interfaces 2017, 9, 42139−42148

Research Article

ACS Applied Materials & Interfaces

Figure 6. (a) Humidity hysteresis characteristics of VS2@MoS2. (Inset) Humidity sensor with a flexible substrate and impedance vs RH curves of VS2@MoS2 at various frequencies. (b) Response and recovery properties of sensors with MoS2, VS2, and VS2@MoS2. These curves were measured from 11 to 95% RH. (c) Long-time stability test of three samples under various humidity. (d) Complex impedance properties of VS2@MoS2 at the different RH.

so we investigated the humidity hysteresis loop of the VS2@ MoS2 composite sensor (Figure 6a) with a small amount of hysteresis (the humidity difference at the same impedance value before and after the cycle). Figure 6b represents the response and recovery curves of the three humidity sensors measured at 100 Hz and RH range from 11 to 95%. A VS2@MoS2 humidity sensor presents a good response/recovery property with response/recovery time as 23 s/13 s, better than VS2. To detect the stability of the device, the change of impedance with time was studied under different humidities every 5 days. As can be seen from Figure 6c, pure MoS2 is stable but not sensitive to humidity with a sensibility [S = Im (11%)/Im (95%)] of 35.6. The pure VS2 humidity sensor has a high sensibility of 3909, whereas after 30 days, the sensibility reduced to 1160.8 which shows a poor stability. For VS2@ MoS2, slight fluctuations occur and the sensibility is kept around 5798.5, indicating that the nanocomposite sensor has good long-term stability. In addition, the sensing response of VS2@MoS2 is better than that of VS2 for humidity detection, indicating that MoS2 is superior in capturing water molecules. The humidity sensing mechanism can be inferred from the complex impedance spectra at different humidities (Figure 6d). At low RH, water molecules are initially adsorbed in the form of chemisorption on the active sites of VS2@MoS2 and dissociated into two hydroxyl groups.35 Then, the water molecules are adsorbed on the surface of a physical layer through the double hydrogen bonds, which cannot move freely, leading to high impedance. With the increase of humidity, a large number of water molecules are adsorbed on the surface of the material physically by a single hydrogen bond and gradually form an approximate liquid water layer. At this point, protons can be passed through neighboring water molecules forming a Grotthuss chain reaction cycle mechanism, which greatly reduces the impedance of the sensor.36

be seen from Figure 5a,b, after 1000 CV cycles, both the onset potential and the current density at the same overpotential of MoS2 decrease apparently, but the HER activity of VS2@MoS2 depresses few. Furthermore, a 20 h galvanostatic measurement at a current density of 20 mA cm−2 was exhibited in Figure 5c. The overpotential required to reach to the certain current density for VS2@MoS2 is more stable than that of VS2 and MoS2 which benefits from the synergistic effect of VS2 and MoS2 in the composite material. MoS2 is compounded with VS2 formatting the metal semiconductor junction. Because of the interaction between the two kinds of materials, the Schottky barrier of metal semiconductor junction decreases, so that near the Fermi level, metal d electrons can be transferred from V to Mo continuously, which improves the efficiency of charge transmission and persistence, and the adsorption of the hydrogen atom on S-VS2 can be passed to S-MoS2 to improve HER efficiency. At the same time, MoS2 with good physical and chemical stability has blocked the contact between vanadium sulfide and oxygen so that it can be in a stable state for a long time. Figure 6 is a simple moisture sensitivity test for the three samples, which proves our inference about the improvement of VS2@MoS2. As can be seen from impedance versus RH curves of VS2@MoS2 at various frequencies (inset of Figure 6a), the VS2@MoS2 humidity sensor shows different properties under different frequencies. The complex impedances are greatly affected by the frequency at low RH and tend to be consistent at high RH. Corresponding to the same RH, the impedance decreases as the frequency increases. This is because the water molecules cannot be polarized at high frequencies, resulting in lower device resistance.34 Besides, the impedances show the best linearity at 100 Hz which is the most suitable frequency for measuring the properties of the VS2@MoS2 humidity sensor. Dehumidification characteristics usually lag behind the absorption process on account of more demands of energy, 42145

DOI: 10.1021/acsami.7b14957 ACS Appl. Mater. Interfaces 2017, 9, 42139−42148

Research Article

ACS Applied Materials & Interfaces Both improvements of VS2@MoS2 nanocomposites for the HER property and humidity characteristics can be contributed to the higher electrical conductivity and the faster electrontransfer process so that the HER rate of the VS2@MoS2 catalyst and humidity sensitivity of the VS2@MoS2 sensor are increased. Also for the HER, not only electrons but also the adsorbed hydrogen atoms are transferred from VS2 to MoS2, which further improves the hydrogen evolution efficiency. In addition, MoS2 grows vertically on the surface of VS2 in the form of thinner nanosheets with more exposed active sites. To analyze the electronic structure of MoS2-edge nanosheets composited with VS2, we consider two types of stacked bilayer structural models as presented in Figure 7a. Without hydrogen adsorbed, we have compared the band structure of these two cases. As shown in Figure 7b, because of the different band dispersion near Fermi level Ef of different stacking cases, we will obtain different electron transports from the valance to conduction band, which will further influence the interband charge transfer related to tunneling toward different interlayer barriers. In fact, this metal−semiconductor Schottky barrier is tunable because the different metal-induced gap states (different band dispersions) will influence the effect of Fermi level pinning which acts as a role of further tuning the Schottky barrier.37,38 To further investigate the hydrogen evolution ability, we calculated the Gibbs free energies of different adsorption sites. It is demonstrated that for the A−B stacking case, when a H atom adsorbed in the inner sites and Mo and S edge sites (shown in Figure 7a), the Gibbs free energies were calculated to be −0.73, 0.14, and 0.31 eV, respectively. In addition, for the A−A stacking case, the corresponding Gibbs free energies to the H atom adsorbed in the inner sites and Mo and S edge sites were calculated to be −0.70, 0.08, and 0.30 eV, respectively. The difference of hydrogen adsorption free energy in the two stacking cases can be mainly attributed to the different interband charge transfer which stems from the interlayer barrier of the metal d electron from VS2 to MoS2. The value of free energies in isolated MoS2 is close to the previous calculation in our group (when a H atom adsorbed in the basal plane and Mo and S edges, the Gibbs free energies were calculated to be −1.17, 0.12, and 0.63 eV, respectively),39 in which both the stacking cases are closer to 0. The electron transport process may be that the electron transfer between metal d electrons attribute to the tunneling effect, and then the electrons from metal d to sulfur electrons followed by transferring to H atoms belong to the charge transfer of neighboring atoms. Moreover, there exists the possibility that the unstable H* may actually transfer from VS2 to MoS2 because the calculated hydrogen adsorption free energy of the VS2 monolayer is 0.21 eV,40 which is larger than that of MoS2-edge nanosheets (inner sites and Mo edge sites) of our cases. It is a generally recognized that the HER performance for the catalyst is considered as good if the Gibbs free energy of adsorbed H is close to zero. Thermodynamically, if the hydrogen adsorption is endothermic, the generation of surface H* would be hindered, whereas if it is too exothermic, the desorption of H* to form H2 would be difficult.41 Considering that the optimal value of free energy should be around 0 eV, which is the ideal state to bind the H atom neither too weakly nor too strongly, we show that the Mo edge sites in the A−A stacking case possess the highest HER activity compared with other adsorbed cases. The calculated value 0.08 eV is comparable to that of the Pt-free catalyst (≈0.09 eV). Figure

Figure 7. (a) Two types of the stacked bilayer structural model and their adsorption configurations. (b) Band structures of the A−B stacking case (left) and the A−A stacking case (right) of the VS2@ MoS2-edge nanosheet. (c) 3D charge density difference surface of the A−A stacking case. The cyan and yellow regions represent the charge depletion and accumulation space, respectively.

7c shows the calculated 3D charge density difference surface of this case. It could be clearly seen that the charge transfer mainly occurs between the adjacent S atoms and the H atom. Therefore, the enhanced charge density of the edge sites mainly contributes to the fast charge transfer in HER activity. 42146

DOI: 10.1021/acsami.7b14957 ACS Appl. Mater. Interfaces 2017, 9, 42139−42148

Research Article

ACS Applied Materials & Interfaces

■

(6) Wang, T.; Chen, S.; Pang, H.; Xue, H.; Yu, Y. MoS2-Based Nanocomposites for Electrochemical Energy Storage. Adv. Sci. 2017, 4, 1600289. (7) Ambrosi, A.; Sofer, Z.; Pumera, M. 2H → 1T phase transition and hydrogen evolution activity of MoS2, MoSe2, WS2 and WSe2 strongly depends on the MX2 composition. Chem. Commun. 2015, 51, 8450−8453. (8) Voiry, D.; Salehi, M.; Silva, R.; Fujita, T.; Chen, M.; Asefa, T.; Shenoy, V. B.; Eda, G.; Chhowalla, M. Conducting MoS2 nanosheets as catalysts for hydrogen evolution reaction. Nano Lett. 2013, 13, 6222−6227. (9) Yu, Y.; Huang, S.-Y.; Li, Y.; Steinmann, S. N.; Yang, W.; Cao, L. Layer-dependent Electrocatalysis of MoS2 for Hydrogen Evolution. Nano Lett. 2014, 14, 553−558. (10) Ambrosi, A.; Sofer, Z.; Pumera, M. Lithium Intercalation Compound Dramatically Influences the Electrochemical Properties of Exfoliated MoS2. Small 2015, 11, 605−612. (11) Xie, J.; Zhang, H.; Li, S.; Wang, R.; Sun, X.; Zhou, M.; Zhou, J.; Lou, X. W.; Xie, Y. Defect-rich MoS2 Ultrathin Nanosheets with Additional Active Edge Sites for Enhanced Electrocatalytic Hydrogen Evolution. Adv. Mater. 2013, 25, 5807−5813. (12) Tao, L.; Duan, X.; Wang, C.; Duan, X.; Wang, S. Plasmaengineered MoS2 Thin-film as an Efficient Electrocatalyst for Hydrogen Evolution Reaction. Chem. Commun. 2015, 51, 7470−7473. (13) Ye, G.; Gong, Y.; Lin, J.; Li, B.; He, Y.; Pantelides, S. T.; Zhou, W.; Vajtai, R.; Ajayan, P. M. Defects Engineered Monolayer MoS2 for Improved Hydrogen Evolution Reaction. Nano Lett. 2016, 16, 1097− 1103. (14) Sun, X.; Dai, J.; Guo, Y.; Wu, C.; Hu, F.; Zhao, J.; Zeng, X.; Xie, Y. Semimetallic Molybdenum Disulfide Ultrathin Nanosheets as an Efficient Electrocatalyst for Hydrogen Evolution. Nanoscale 2014, 6, 8359−8367. (15) Yang, L.; Fu, Q.; Wang, W.; Huang, J.; Huang, J.; Zhang, J.; Xiang, B. Large-area Synthesis of Monolayered MoS2(1‑x)Se2x with a Tunable Band Gap and Its Enhanced Electrochemical Catalytic Activity. Nanoscale 2015, 7, 10490−10497. (16) McAteer, D.; Gholamvand, Z.; McEvoy, N.; Harvey, A.; O’Malley, E.; Duesberg, G. S.; Coleman, J. N. Thickness Dependence and Percolation Scaling of Hydrogen Production Rate in MoS2 Nanosheet and Nanosheet−Carbon Nanotube Composite Catalytic Electrodes. ACS Nano 2016, 10, 672−683. (17) Zhang, B.; Liu, J.; Wang, J.; Ruan, Y.; Ji, X.; Xu, K.; Chen, C.; Wan, H.; Miao, L.; Jiang, J. Interface Engineering: the Ni(OH)2/MoS2 Heterostructure for Highly Efficient Alkaline Hydrogen Evolution. Nano Energy 2017, 37, 74−80. (18) Li, Y.; Wang, H.; Xie, L.; Liang, Y.; Hong, G.; Dai, H. MoS2 Nanoparticles Grown on Graphene: An Advanced Catalyst for the Hydrogen Evolution Reaction. J. Am. Chem. Soc. 2011, 133, 7296− 7299. (19) Yang, L.; Zhou, W.; Hou, D.; Zhou, K.; Li, G.; Tang, Z.; Li, L.; Chen, S. Porous Metallic MoO2-supported MoS2 Nanosheets for Enhanced Electrocatalytic Activity in the Hydrogen Evolution reaction. Nanoscale 2015, 7, 5203−5208. (20) Liao, L.; Zhu, J.; Bian, X.; Zhu, L.; Scanlon, M. D.; Girault, H. H.; Liu, B. MoS2 Formed on Mesoporous Graphene as a Highly Active Catalyst for Hydrogen Evolution. Adv. Funct. Mater. 2013, 23, 5326− 5333. (21) Chen, W.; Santos, E. J. G.; Zhu, W.; Kaxiras, E.; Zhang, Z. Tuning the Electronic and Chemical Properties of Monolayer MoS2 Adsorbed on Transition Metal Substrates. Nano Lett. 2013, 13, 509− 514. (22) Tsai, C.; Abild-Pedersen, F.; Nørskov, J. K. Tuning the MoS2 Edge-site Activity for Hydrogen Evolution via Support Interactions. Nano Lett. 2014, 14, 1381−1387. (23) Feng, J.; Sun, X.; Wu, C.; Peng, L.; Lin, C.; Hu, S.; Yang, J.; Xie, Y. Metallic Few-layered VS2 Ultrathin Nanosheets: High Twodimensional Conductivity for In-plane Supercapacitors. J. Am. Chem. Soc. 2011, 133, 17832−17838.

CONCLUSIONS In summary, we have reported a VS2@MoS2 nanomaterial, with MoS2 nanosheets growing vertically on VS2 nanoflowers via a modified two-step hydrothermal method for the first time. Owing to the high conductivity and rich exposed active edge sites, the VS2@MoS2 nanocatalyst exhibited an excellent HER performance with a low onset potential, large cathodic currents, and a small Tafel slope. Meanwhile, good structural and physicochemical stability of the nanocomposite, together with the high conductivity resulted in improved humidity characteristics. We believe that the VS2@MoS2 nanomaterial reported in this work may also be applied to other fields, and this thought of choosing different materials to synthesize can be contributed to other material systems.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b14957. S L-edge NEXAFS spectra of the VS2@MoS2 hybrid and pure MoS2, UPS of (a) MoS2 and (b) VS2, XPS spectra of Mo 3d for pure MoS2 and VS2@MoS2 composites, HRTEM of the VS2@MoS2 hybrid, three-electrode system equipment for the HER test with VS2@MoS2 as the working electrode (PDF)

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +86 21 54345198. ORCID

Ke Yu: 0000-0002-9920-2555 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors acknowledge financial support from the NSF of China (grant nos. 61574055, 61474043), the Open Project Program of Key Laboratory of Polar Materials and Devices, MOE (grant no. KFKT20140003), East China Normal University, and the Catalysis and Surface Science Endstation in the National Synchrotron Radiation Laboratory (NSRL) in Hefei, China.

■

REFERENCES

(1) Norskov, J. K.; Christensen, C. H. Toward Efficient Hydrogen Production at Surfaces. Science 2006, 312, 1322−1323. (2) Voiry, D.; Yang, J.; Chhowalla, M. Recent Strategies for Improving the Catalytic Activity of 2D TMD Nanosheets Toward the Hydrogen Evolution Reaction. Adv. Mater. 2016, 28, 6197−6206. (3) Wang, X.; Wang, P.; Wang, J.; Hu, W.; Zhou, X.; Guo, N.; Huang, H.; Sun, S.; Shen, H.; Lin, T.; Tang, M.; Liao, L.; Jiang, A.; Sun, J.; Meng, X.; Chen, X.; Lu, W.; Chu, J. Ultrasensitive and Broadband MoS2 Photodetector Driven by Ferroelectrics. Adv. Mater. 2015, 27, 6575−6581. (4) Wang, J.; Zou, X.; Xiao, X.; Xu, L.; Wang, C.; Jiang, C.; Ho, J. C.; Wang, T.; Li, J.; Liao, L. Floating Gate Memory-based Monolayer MoS2 Transistor with Metal Nanocrystals Embedded in the Gate Dielectrics. Small 2015, 11, 208−213. (5) Jaramillo, T. F.; Jorgensen, K. P.; Bonde, J.; Nielsen, J. H.; Horch, S.; Chorkendorff, I. Identification of Active Edge Sites for Electrochemical H2 Evolution from MoS2 Nanocatalysts. Science 2007, 317, 100−102. 42147

DOI: 10.1021/acsami.7b14957 ACS Appl. Mater. Interfaces 2017, 9, 42139−42148

Research Article

ACS Applied Materials & Interfaces (24) Feng, J.; Peng, L.; Wu, C.; Sun, X.; Hu, S.; Lin, C.; Dai, J.; Yang, J.; Xie, Y. Giant Moisture Responsiveness of VS2 Ultrathin Nanosheets for Novel Touchless Positioning Interface. Adv. Mater. 2012, 24, 1969−1974. (25) Chia, X.; Ambrosi, A.; Lazar, P.; Sofer, Z.; Pumera, M. Electrocatalysis of Layered Group 5 Metallic Transition Metal Dichalcogenides (MX2, M = V, Nb, and Ta; X = S, Se, and Te). J. Mater. Chem. A 2016, 4, 14241−14253. (26) Liu, Y.; Wu, J.; Hackenberg, K. P.; Zhang, J.; Wang, Y. M.; Yang, Y.; Keyshar, K.; Gu, J.; Ogitsu, T.; Vajtai, R.; Lou, J.; Ajayan, P. M.; Wood, B. C.; Yakobson, B. I. Self-optimizing, Highly Surface-active Layered Metal Dichalcogenide Catalysts for Hydrogen Evolution. Nat. Energy 2017, 2, 17127. (27) Liang, H.; Shi, H.; Zhang, D.; Ming, F.; Wang, R.; Zhuo, J.; Wang, Z. Solution Growth of Vertical VS2 Nanoplate Arrays for Electrocatalytic Hydrogen Evolution. Chem. Mater. 2016, 28, 5587− 5591. (28) Ding, S.; Zhang, D.; Chen, J. S.; Lou, X. W. Facile Synthesis of Hierarchical MoS2 Microspheres Composed of Few-layered Nanosheets and Their Lithium Storage Properties. Nanoscale 2012, 4, 95− 98. (29) Wan, Z.; Shao, J.; Yun, J.; Zheng, H.; Gao, T.; Shen, M.; Qu, Q.; Zheng, H. Core-shell Structure of Hierarchical Quasi-hollow MoS2 Microspheres Encapsulated Porous Carbon as Stable Anode for Li-ion Batteries. Small 2014, 10, 4975−4981. (30) Zhang, D.; Sun, Y.; Li, P.; Zhang, Y. Facile Fabrication of MoS2Modified SnO2 Hybrid Nanocomposite for Ultrasensitive Humidity Sensing. ACS Appl. Mater. Interfaces 2016, 8, 14142−14149. (31) Cao, F.; Hu, W.; Zhou, L.; Shi, W.; Song, S.; Lei, Y.; Wang, S.; Zhang, H. 3D Fe3S4 Flower-like Microspheres: High-yield Synthesis via a Biomolecule-assisted Solution Approach, Their Electrical, Magnetic and Electrochemical Hydrogen Storage Properties. Dalton Trans. 2009, 9246−9252. (32) Zhang, X.; Ding, P.; Sun, Y.; Wang, Y.; Wu, Y.; Guo, J. Shellcore MoS2 Nanosheets@Fe3O4 Sphere Heterostructure with Exposed Active Edges for Efficient Electrocatalytic Hydrogen Production. J. Alloys Compd. 2017, 715, 53−59. (33) Li, F.; Zhang, L.; Li, J.; Lin, X.; Li, X.; Fang, Y.; Huang, J.; Li, W.; Tian, M.; Jin, J.; Li, R. Synthesis of Cu−MoS2/rGO Hybrid as Non-noble Metal Electrocatalysts for the Hydrogen Evolution Reaction. J. Power Sources 2015, 292, 15−22. (34) Wang, J.; Wan, H.; Lin, Q. Properties of a Nanocrystalline Barium Titanate on Silicon Humidity Sensor. Meas. Sci. Technol. 2003, 14, 172−175. (35) He, Y.; Zhang, T.; Zheng, W.; Wang, R.; Liu, X.; Xia, Y.; Zhao, J. Humidity Sensing Properties of BaTiO3 Nanofiber Prepared via Electrospinning. Sens. Actuators, B 2010, 146, 98−102. (36) Lu, Y.; Wang, Z.; Yuan, S.; Shi, L.; Zhao, Y.; Deng, W. Microwave-hydrothermal Synthesis and Humidity Sensing Behavior of ZrO2 Nanorods. RSC Adv. 2013, 3, 11707−11714. (37) Liu, Y.; Stradins, P.; Wei, S.-H. Van der Waals Metalsemiconductor Junction: Weak Fermi Level Pinning Enables Effective Tuning of Schottky Barrier. Sci. Adv. 2016, 2, No. e1600069. (38) Liu, Y.; Xiao, H.; Goddard, W. A. Schottky-Barrier-Free Contacts with Two-Dimensional Semiconductors by Surface-Engineered MXenes. J. Am. Chem. Soc. 2016, 138, 15853−15856. (39) Guo, B.; Yu, K.; Li, H.; Song, H.; Zhang, Y.; Lei, X.; Fu, H.; Tan, Y.; Zhu, Z. Hollow Structured Micro/Nano MoS2 Spheres for High Electrocatalytic Activity Hydrogen Evolution Reaction. ACS Appl. Mater. Interfaces 2016, 8, 5517−5525. (40) Tsai, C.; Chan, K.; Nørskov, J. K.; Abild-Pedersen, F. Theoretical Insights into the Hydrogen Evolution Activity of Layered Transition Metal Dichalcogenides. Surf. Sci. 2015, 640, 133−140. (41) Xing, J.; Jiang, H. B.; Chen, J. F.; Li, Y. H.; Wu, L.; Yang, S.; Zheng, L. R.; Wang, H. F.; Hu, P.; Zhao, H. J.; Yang, H. G. Active Sites on Hydrogen Evolution Photocatalyst. J. Mater. Chem. A 2013, 1, 15258−15264.

42148

DOI: 10.1021/acsami.7b14957 ACS Appl. Mater. Interfaces 2017, 9, 42139−42148